GET THEE A MITER SAW FENCE AND SOME STOPS
Work off the INSIDE dimensions for framing.
Cut the initial stock long
Miter one corner of each pair
From the "short side" of the first miter cut
measure and mark the INSIDE of the second
miter cut, preferably with a marking knife,
on the face you'll make the second cut on.
That'll be the "underside" relative to the
side facing up for the initial miter cut.
With the saw still at the first cut angle, flip
the part end for end, with your scribed line
towards you so you can align the part such
that a "keeper side of the cut" tooth's "keeper
side of the cut" is on your line.
Set a stop to the other end of the "keeper"
Make the second cut on the first of the pair
for frame parts one.
Place the second part of pair one to the stop,
cut the first miter, flip the part end for end,
registering the first mitered corner end
against the stop and make the seccond miter
cut. NOTE: the stop must be square to the
fence and the sharp outer corner of the
first mitered cut has to be against the stop
NOT BEHIND IT - EVEN JUST A LITTLE BIT.
Repeat the process for the second pair of
NOTE: If the cabinet edges aren't square
things get a LOT trickier
Picture framers have a slick tool that uses
a "guillotine" (sp?) knife to fine tune the
miterer corners (see "Lion Trimmer" or
"Potlach") by slicing off see through shavings.
Hard to justify that tool purchase unless you
plan on doing a LOT of mitered frames.